Firing rate interactions among human orbicularis oris motor units

Proprioceptive innervation of the human lips

The objective of this study was to analyze the proprioceptive innervation of human lips, especially of the orbicularis oris muscle, since it is classically accepted that facial muscles lack typical proprioceptors, that is, muscle spindles, but recently this has been doubted. Upper and lower human lips (n = 5) from non-embalmed frozen cadavers were immunostained for detection of S100 protein (to identify nerves and sensory nerve formations), myosin heavy chain (to label muscle fibers within muscle spindles), and the mechano-gated ion channel PIEZO2. No muscle spindles were found, but there was a high density of sensory nerve formations, which were morphologically heterogeneous, and in some cases resemble Ruffini-like and Pacinian sensory corpuscles. The axons of these sensory formations displayed immunoreactivity for PIEZO2. Human lip muscles lack typical proprioceptors but possess a dense sensory innervation which can serve the lip proprioception.

The effects of aging on the distribution and strength of correlated neural inputs to postural muscles during unperturbed bipedal stance

The present study investigated the effects of aging on the distribution of common descending neural drives to main postural muscles acting on the ankle, knee, hip, and lower trunk. The presence, distribution, and strength of these drives were assessed using intermuscular coherence estimations at a low-frequency band (0-55 Hz). Ten healthy older adults (68.7 ± 3.5 years) with no recent history of falls and ten healthy younger adults (26.8 ± 2.7 years) performed bipedal stances with eyes either opened or closed. Electromyographic (EMG) signals of six postural muscles were recorded. Estimations of intermuscular coherence were obtained from fifteen muscle pairs and four muscle groups. In general, single-pair and pooled coherence analyzes revealed significant levels of signal synchronization within 1-10 Hz. Significant common drives to anterior, posterior, and antagonist muscle groups were observed for both cohorts of participants. However, older participants showed significantly stronger EMG-EMG synchronization in the frequency domain compared to younger participants. It seems that age-related sarcopenia, visual-vestibular-proprioceptive decline, cortical activation increase, presynaptic inhibition modulation decrease, and co-contraction increase had a major impact on strengthening the common drives to the aforementioned muscle groups. Differently from young adults, the absence of visual inputs did not reduce the magnitude of signal synchronization in older adults. These results suggest that the aging central nervous system seems to organize similar arrangements of common drives to postural antagonist muscles at different joints, and to postural muscles pushing the body either forward or backward when visual information is not available.

The reliability of surface EMG derived motor unit variables

Motor unit (MU) recordings obtained from surface electromyography (sEMG) decomposition are used to investigate the neural control of muscle in response to interventions, but our understanding of the longer-term reliability of MU variables is limited. This study examined the reliability of MU variables in the flexor carpi radialis (FCR) and tibialis anterior (TA) over a three-month period. Forty college-aged participants completed isometric wrist flexion (n = 20) and dorsiflexion (n = 20). There were 3 maximal isometric voluntary contractions (MVC) and 3 ramp contractions to 60% of MVC on four separate sessions separated by a total of 13 weeks. Intraclass correlation coefficients (ICC) were calculated from a fully nested ANOVA model. Maximal force was highly reliable (ICC = 0.94-0.99). The ICC values ranged from 0.49 to 0.92 for the FCR MU variables and from 0.58 to 0.96 for the TA MU variables. All MU variables exhibited a high degree of stability of means across test session and consistency within subjects, with the exception of the number of MUs detected in the TA. Poor ICC values did not reflect poor reliability but rather, convergence towards a narrow range of physiologically normal values. Surface EMG decomposition of a large population of MUs showed no differences in common drive between FCR (0.273) and for the TA (0.267) across test sessions. Forty percent of the sampled MUs in both muscles had a common drive of 0.30 or greater, which provides indirect support for the validity of the decompositions. MU variables may be used to monitor adaptations to a longer-term intervention study.

The difficulty of the postural control task affects multi-muscle control during quiet standing

The aim of this study was to compare the electromyographic (EMG) coherence between the lower limb and the core muscles when carrying out two postural tasks at different difficulty levels. EMG was recorded in 20 healthy male subjects while performing two independent quiet standing tasks. The first one involved a bipedal stance with the eyes open, while the second consisted of a dominant unipedal stance also with the eyes open. The obtained EMG signals were analysed by computing estimations of EMG–EMG coherence between muscle pairs, both singly (single-pair estimations) and combined (pooled estimations). Pooled and single coherence of anterior, posterior, core, antagonist and mixed pairs of muscles were significant in the 0–5 Hz frequency band. The results indicate that core and antagonist muscle groups, such as the anterior and posterior muscles, share low-frequency neural inputs (0–5 Hz) which could be responsible of the M-modes assembly. The core muscles could therefore provide the necessary synergy to maintain spine stability during the balancing exercise. Finally, differences in EMG–EMG coherence suggest that the muscle synergies formed during unipedal stance tasks are different from those established during bipedal stance.

Motor unit firing pattern, synchrony and coherence in a deafferented patient

The firing of spinal motoneurons (MNs) is controlled continuously by inputs from muscle, joint and skin receptors. Besides altering MN synaptic drive, the removal of these inputs is liable to alter the synaptic noise and, thus, the variability of their tonic activity. Sensory afferents, which are a major source of common and/or synchronized inputs shared by several MNs, may also contribute to the coupling in the time and frequency domains (synchrony and coherence, respectively) observed when cross-correlation and coherence analyses are applied to the discharges of MN pairs. Surprisingly, no consistent changes in firing frequency, nor in synchrony and coherence were reported to affect the activity of 3 pairs of motor units (MUs) tested in a case of sensory polyradiculoneuropathy (SPRNP), leading to an irreversible loss of large diameter sensory afferents (Farmer et al., 1993). Such a limited sample, however, precludes a definite conclusion about the actual impact that a chronic loss of muscle and cutaneous afferents may have on the firing properties of human MUs. To address this issue, the firing pattern of 92 MU pairs was analyzed at low contraction force in a case of SPRNP leading similarly to a permanent loss of proprioceptive inputs. Compared with 8 control subjects, MNs in this patient tended to discharge with slightly shorter inter-spike intervals but with greater variability. Synchronous firing tended to occur more frequently with a tighter coupling in the patient. There was no consistent change in coherence in the 15–30 Hz frequency range attributed to the MN corticospinal drive, but a greater coherence was observed below 5 Hz and between 30 and 60 Hz in the patient. The possible origins of the greater irregularity in MN tonic discharges, the tighter coupling of the synchronous firing and the changes in coherence observed in the absence of proprioceptive inputs are discussed.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Neuromuscular mechanisms and neural strategies in the control of time-varying muscle contractions

The organization of the neural input to motoneurons that underlies time-varying muscle force is assumed to depend on muscle transfer characteristics and neural strategies or control modes utilizing sensory signals. We jointly addressed these interlinked, but previously studied individually and partially, issues for sinusoidal (range 0.5–5.0 Hz) force-tracking contractions of a human finger muscle. Using spectral and correlation analyses of target signal, force signal, and motor unit (MU) discharges, we studied 1) patterns of such discharges, allowing inferences on the motoneuronal input; 2) transformation of MU population activity (EMG) into quasi-sinusoidal force; and 3) relation of force oscillation to target, carrying information on the input's organization. A broad view of force control mechanisms and strategies emerged. Specifically, synchronized MU and EMG modulations, reflecting a frequency-modulated motoneuronal input, accompanied the force variations. Gain and delay drops between EMG modulation and force oscillation, critical for the appropriate organization of this input, occurred with increasing target frequency. According to our analyses, gain compensation was achieved primarily through rhythmical activation/deactivation of higher-threshold MUs and secondarily through the adaptation of the input's strength expected during tracking tasks. However, the input's timing was not adapted to delay behaviors and seemed to depend on the control modes employed. Thus, for low-frequency targets, the force oscillation was highly coherent with, but led, a target, this timing error being compatible with predictive feedforward control partly based on the target's derivatives. In contrast, the force oscillation was weakly coherent, but in phase, with high-frequency targets, suggesting control mainly based on a target's rhythm.

Motor unit recruitment and proprioceptive feedback decrease the common drive

It has been documented that concurrently active motor units fire under the control of a common drive. That is, the firing rates show high correlation with near-zero time lag. This degree of correlation has been found to vary among muscles and among contractions performed at different force levels in the same muscle. This study provides an explanation indicating that motor units recruited during a contraction cause an increase in the variation (SD) and a decrease in the degree (amplitude) of the correlation of the firing rates. The degree of correlation is lower in muscles having greater spindle density. This effect appears to be mediated by the proprioceptive feedback from the spindles and possibly the Golgi tendon organs. Muscle spindles in particular respond to the mechanical excitation of the nonfused muscle fibers and provide a discordant excitation to the homonymous motoneurons, resulting in a decrease in the correlation of the firing rates of motor units. The implication of this work is that the decreased correlation of the firing rates in some muscles is not necessarily an indication of a decreased common drive from the CNS, but rather an inhibitory influence of the proprioceptive feedback from the peripheral nervous system. This explanation is useful for understanding various manifestations of the common drive reported in the literature.

Motor Unit Synchronization Is Increased in Biceps Brachii After Exercise-Induced Damage to Elbow Flexor Muscles

The purpose of this study was to determine the effect of eccentric exercise on correlated motor unit discharge (motor unit synchronization and coherence) during low-force contractions of the human biceps brachii muscle. Eight subjects (age, 25 ± 7 yr) performed three tasks involving isometric contraction of elbow flexors while EMG (surface and intramuscular) records were obtained from biceps brachii. Tasks were 1) maximum voluntary contraction (MVC); 2) constant-force contraction at various submaximal targets; and 3) sustained discharge of pairs of concurrently active motor units for 2–5 min. These tasks were performed before, immediately after, and 24 h after fatiguing eccentric exercise. MVC force declined 46% immediately after eccentric exercise and remained depressed (31%) 24 h later, which is indicative of muscle damage. For the constant-force task, biceps brachii EMG (∼100% greater) and force fluctuations (∼75% greater) increased immediately after exercise, and both recovered by ∼50% 24 h later. Motor unit synchronization, quantified by cross-correlation of motor unit pairs during low-force (1–26% MVC) contractions, was 30% greater immediately after ( n = 105 pairs) and 24 h after exercise ( n = 92 pairs) compared with before exercise ( n = 99 pairs). Similarly, motor unit coherence at low (0–10 Hz) frequencies was 20% greater immediately after exercise and 34% greater 24 h later. These results indicate that the series of events leading to muscle damage from eccentric exercise alters the correlated behavior of human motor units in biceps brachii muscle for ≥24 h after the exercise.

Coherent Motor Unit Rhythms in the 6–10 Hz Range During Time-Varying Voluntary Muscle Contractions: Neural Mechanism and Relation to Rhythmical Motor Control

In quasi-sinusoidal (0.5–3.0 Hz) voluntary muscle contractions, we studied the 6- to 10-Hz motor unit (MU) firing synchrony and muscle force oscillation with emphasis on their neural substrate and relation to rhythmical motor control. Our analyses were performed on data from 121 contractions of a finger muscle in 24 human subjects. They demonstrate that coherent 6- to 10-Hz components of MU discharges coexist with carrier components and coherent modulation components underlying the voluntary force variations. The 6- to 10-Hz synchrony has the frequency of the tremor synchrony in steady contractions and is also widespread and in-phase. Its strength ranges from very small to very large (MU/MU coherence >0.50) among contractions; moreover, it is not related to the contraction parameters, in accord with the notion of a distinct 6- to 10-Hz synaptic input to the MUs. Unlike the coherent MU modulations and the voluntary force variations, the in-phase 6- to 10-Hz MU components are suppressed or even eliminated during ischemia, while the respective force component is drastically reduced. These findings agree with the widely assumed supraspinal origin of the MU modulations, but they also strongly suggest a key role for muscle spindle feedback in the generation of the 6- to 10-Hz synaptic input. They therefore provide important information for the study of generators of the 6- to 10-Hz rhythm which subserves the postulated rhythmical control and is manifested as force and movement components. Moreover, they argue for a participation of oscillating spinal stretch reflex loops in the rhythm generation, possibly in interaction with supraspinal oscillators.

Factors Affecting the Common Modulation of Bilateral Motor Unit Discharge in Human Soleus Muscles

The purpose of this study was to determine the factors that influence the co-modulation of motor unit discharge rate in soleus muscles of both legs during upright standing. Single motor units were recorded from the left and right soleus muscles under three experimental conditions: standing quietly with the eyes open and closed, standing with the eyes closed while vibration was applied to one Achilles tendon, and swaying voluntarily or producing variable low-force isometric contractions at a frequency of 0.05 Hz. Correlations in motor unit discharge rate between left and right soleus motor units were assessed using common drive analysis. The results showed that common drive to motoneurons of the two muscles did not differ between standing with the eyes open or closed, but there was an order effect with the second task having significantly lower common drive than the first. Common drive was also significantly lower when vibration was applied to one leg compared with when no vibration was applied. Common drive was higher as subjects swayed anteriorly as compared with when they swayed posteriorly. There were no significant differences in common drive across phases of the variable isometric force contraction. Common drive was higher during voluntary sway than during variable force production; both of these values were significantly lower than those derived from the quiet standing task. These results suggest that proprioceptive and sub-cortical inputs contribute to the co-modulation of the firing rate of soleus motor unit pairs of the left and right leg during standing posture.

Discharge rate during low-force isometric contractions influences motor unit coherence below 15 Hz but not motor unit synchronization

The purpose of the study was to determine whether pairs of motor units that discharge action potentials at different rates during isometric contractions exhibit different levels of motor unit synchronization or coherence. Twelve subjects (28.6 +/- 6.1 years) performed isometric contractions at target forces slightly above the recruitment threshold (1.02-20.9%) of an isolated motor unit. Based on audio feedback, subjects maintained a relatively constant discharge rate of the isolated unit for about 80 s. Intramuscular electrodes were used to record the discharge of 47 pairs of motor units at rates that ranged from 8.07 to 13.6 pps. Correlated discharge between pairs of motor units was quantified with the common input strength (CIS) index, k' index, and coherence spectrum. Greater discharge rates across pairs of motor units were predicted (R2 = 0.36, P < 0.001) by higher coherence from 8 to 13 Hz (r = -0.52) and lower coherence from 0 to 4 Hz (r = 0.37). Indexes of motor unit synchronization (CIS and k') were strongly associated with motor unit coherence from 16 to 32 Hz (CIS: R2 = 0.63; k': R2 = 0.4; P = 0.001). The CIS index of motor unit synchronization and the motor unit coherence from 16 to 32 Hz did not vary with discharge rate. In contrast, the k' index of motor unit synchronization declined with discharge rate (r2 = 0.20, P = 0.001). Furthermore, greater discharge rates across pairs of motor units were accompanied by higher motor unit coherence in the 8-13 Hz band and lower motor unit coherence in the 0-4 Hz band. These results demonstrate that differences in discharge rate between pairs of motor units in first dorsal interosseus during low-force, isometric contractions were associated with modulation of the correlation in the discharge times of the two motor units at frequencies less than 15 Hz.

Topographical Characteristics of Motor Units of the Lower Facial Musculature Revealed by Means of High-Density Surface EMG

The objective of this study was to systematically characterize motor units (MUs) of the musculature of the lower face. MU endplate positions and principal muscle fiber orientations relative to facial landmarks were identified. This was done by the analysis of motor unit action potentials (MUAPs) in the surface electromyogram. Thirteen specially trained, healthy subjects performed selective contractions of the depressor anguli oris, depressor labii inferioris, mentalis, and orbicularis oris inferior muscles. Signals were recorded using recently developed, 0.3-mm thin and flexible high-density surface electromyography (sEMG) grids (120 channels). For each subject and each muscle and for different low contraction levels, representative MUAPs ("MU fingerprints") were extracted from the raw sEMG data according to their spatiotemporal amplitude characteristics. We then topographically characterized the lower facial MUs' endplate zones and main muscle fiber orientations on the individual faces of the subjects. These topographical MU parameters were spatially warped to correct for the different sizes and shapes of the faces of individual subjects. This electrophysiological study revealed a distribution of the lower facial MU endplates in more or less restricted, distinct clusters on the muscle often with eccentric locations. The results add substantially to the basic neurophysiologic and anatomical knowledge of the complex facial muscle system. They can also be used to establish objective guidelines for placement of conventional (surface or needle) EMG electrodes as well as for clinical investigations on neuromuscular diseases affecting the facial musculature. The localized endplate positions may also indicate optimal locations for botulinum toxin injection in the face.

A simulation study to examine the effect of common motoneuron inputs on correlated patterns of motor unit discharge

The influence of common oscillatory inputs to the motoneuron pool on correlated patterns of motor unit discharge was examined using model simulations. Motor unit synchronization, in-phase fluctuations in mean firing rates known as 'common drive', and the coefficient of variation of the muscle force were examined as the frequency and amplitude of common oscillatory inputs to the motoneuron pool were varied. The amount of synchronization, the peak correlation between mean firing rates and the coefficient of variation of the force varied with both the frequency and amplitude of the common input signal. Values for 'common drive' and the force coefficient of variation were highest for oscillatory inputs at frequencies less than 5 Hz, while synchronization reached a maximum when the frequency of the common input was close to the average motor unit firing rate. The frequency of the common input signal for which the highest levels of synchronization were observed increased as motoneuron firing rates increased in response to higher target force levels. The simulation results suggest that common low-frequency oscillations in motor unit firing rates and short-term synchronization result from oscillatory activity in different bands of the frequency spectrum of shared motoneuron inputs. The results also indicate that the amount of synchronization between motor unit discharges depends not only on the amplitude of the shared input signal, but also on its frequency in relation to the present firing rates of the individual motor units.

Frequency Modulation of Motor Unit Discharge Has Task-Dependent Effects on Fluctuations in Motor Output

The rate of change in the fluctuations in motor output differs during the performance of fatiguing contractions that involve different types of loads. The purpose of this study was to examine the contribution of frequency modulation of motor unit discharge to the fluctuations in the motor output during sustained contractions with the force and position tasks. In separate tests with the upper arm vertical and the elbow flexed to 1.57 rad, the seated subjects maintained either a constant upward force at the wrist (force task) or a constant elbow angle (position task). The force and position tasks were performed in random order at a target force equal to 3.6 ± 2.1% (mean ± SD) of the maximal voluntary contraction (MVC) force above the recruitment threshold of an isolated motor unit from the biceps brachii. Each subject maintained the two tasks for an identical duration (161 ± 93 s) at a mean target force of 22.4 ± 13.6% MVC. As expected, the rate of increase in the fluctuations in motor output (force task: SD for detrended force; position task: SD for vertical acceleration) was greater for the position task than the force task ( P < 0.001). The amplitude of the coefficient of variation (CV) and the power spectra for motor unit discharge were similar between tasks ( P > 0.1) and did not change with time ( P > 0.1), and could not explain the different rates of increase in motor output fluctuations for the two tasks. Nonetheless, frequency modulation of motor unit discharge differed during the two tasks and predicted ( P < 0.001) both the CV for discharge rate (force task: 1–3, 12–13, and 14–15 Hz; position task: 0–1, and 1–2 Hz) and the fluctuations in motor output (force task: 5–6, 9–10, 12–13, and 14–15 Hz; position task: 6–7, 14–15, 17–19, 20–21, and 23–24 Hz). Frequency modulation of motor unit discharge rate differed for the force and position tasks and influenced the ability to sustain steady contractions.

Enhanced motor unit rate coding with improvements in a force-matching task

These data describe improved modulation of discharge rates (rate coding) of first dorsal interosseous motor units throughout the acquisition of a complex force-matching skill involving isometric index finger abduction. In each of 15 consecutive trials, subjects attempted to match their force to a trajectory consisting of the sum of two sine waves (0.15 and 0.5 Hz) and random oscillations (overall mean force level approximately 20% MVC). Reductions in root-mean-square (RMS) error of each subject's force relative to the trajectory indicated substantial improvements in force-matching ability (F = 33.8, p < 0.001). With the acquisition of this new skill, there was increased amplitude modulation of muscular force near both dominant frequencies of the force-matching trajectory (F = 10.6, p = 0.008). The standard deviation and coefficient of variation of motor unit inter-spike intervals both decreased with improved performance indicating a general reduction in the amplitude of firing rate modulations (SD: F = 18.69, p = 0.001; CV: F = 43.6, p < 0.001). After skill acquisition, there was decreased firing rate modulation outside of the two dominant frequencies and increased amplitude of firing rate modulation at the higher of the two dominant frequencies (0.5 Hz, F = 8.23, p = 0.015). These findings indicate that improved precision of rate coding was a contributor to the acquisition of the new force-matching task. That the change in rate coding was frequency dependent suggests that factors other than frequency coding may contribute to the improved force matching at 0.15 Hz.

The 1- to 2-Hz oscillations in muscle force are exacerbated by stress, especially in older adults

Although force fluctuations during a steady contraction are often heightened in old adults compared with young adults and are enhanced in young adults during the stress response, the mechanisms underlying the augmentation are uncertain. The purpose of the study was to compare the effect of a stressor on the plasma concentrations of selected stress hormones and on the force fluctuations experienced by young and old adults during the performance of a precision grip. Thirty-six men and women (19–86 yr) participated in a protocol that comprised anticipatory (30 min), stressor (15 min), and recovery periods (25 min). The stressor was a series of noxious electrical stimuli applied to the dorsal surface of the left hand. Subjects sustained a pinch-grip force with the right hand at 2% of the maximal voluntary contraction force. The fluctuations in pinch-grip force, the interference electromyogram (EMG) of six muscles, and the spectra for the force and EMG were quantified across the 70-min protocol. The stressor increased the force fluctuations, largely due to an enhancement of the power at 1–2 Hz in the force spectrum ( r2 = 0.46). The effect was greatest for the old adults compared with young and middle-aged adults. The plasma concentrations of the stress hormones (adrenocorticotropin, epinephrine, and norepinephrine) were elevated to similar levels for all three age groups, and the changes were not associated with modulation of the force fluctuations. Furthermore, the heightened EMG activity exhibited by the old adults during all periods was not related to the changes in the force fluctuations or the 1- to 2-Hz force oscillations. The absence of a change in the mean pinch-grip force during the protocol and the lack of an association between elevation of the plasma concentrations for the stress hormones and modulation of the force fluctuations suggest that the enhanced force fluctuations caused by the stressor was due to an increase in the low-frequency output of the spinal motor neurons.

Aging and Rhythmical Force Output: Loss of Adaptive Control of Multiple Neural Oscillators

The current study examined the influence of aging on the oscillatory activity of a population of motor units during rhythmical force production. Previously, it has been shown that aging humans have greater low-frequency and less high-frequency electromyographic (EMG) activity during constant and slow ramp force contractions. We hypothesized that more rapid force contractions would reverse the established finding of reduced high- and greater low-frequency EMG activity to greater high- and reduced low-frequency EMG activity in older adults. Intramuscular EMG activity and effector force were recorded while 45 human subjects (20–31 and 60–88 yr of age) rhythmically produced force at four distinct frequencies (1–4 Hz) and two force levels (5 and 25% maximal voluntary contraction). Spectral and coherence analyses were performed on the force output and EMG activity. In the 3- and 4-Hz targets, the older adults had greater 35- to 50-Hz and reduced 0- to 5-Hz EMG activity compared with the young adults. There was greater EMG-force coherence in the 0- to 5-Hz bandwidth for the young subjects. No systematic age difference in the phase relationship between the EMG and force signals were found. Higher frequency force contractions reversed the previously established aging differences in the relative contribution of low- and high-frequency EMG activity. Thus the frequency properties of the task goals channel the relative contribution of low and high EMG activity. Furthermore, it is proposed that aging humans lose the adaptive capability to coordinate the excitatory and inhibitory activity of multiple neural oscillators.

Time-dependent structure in the discharge rate of human motor units

OBJECTIVES

The aim of this study was to examine the influence of visual and motor processes on the deterministic and stochastic structure of force output and motor unit discharge variability.

METHODS

Young adult subjects produced continuous, isometric force at 3, 6, 12, and 24% of their maximal voluntary contraction at low and high visual gain levels through abduction of the index finger. Force and fine-wire intramuscular electromyography were recorded.

RESULTS

There was a linear increase in discharge irregularity with increases in the mean motor unit discharge rate (8-30 Hz). Recurrence analysis showed that the percentage of deterministic structure in discharge variability remained high, but decreased linearly with increased motor unit discharge rate. Surrogate analyses confirmed that the motor unit discharge variability was inconsistent with an uncorrelated and linearly correlated Gaussian noise process. Spectral analysis revealed that both the force output and the mean time-varying motor unit discharge time series had a dominant frequency of 0-2 Hz. Visual feedback gain did not affect the individual motor unit discharge patterns.

CONCLUSIONS

The motor unit discharge rate has deterministic time-dependent structure. The motor unit discharge rate is modulated at multiple time scales likely by pre- and post-synaptic induced fluctuations from spinal level pathways impinging on the motor neuron.

Common drive in motor units of a synergistic muscle pair

The interaction among the motor units of the extensor carpi radialis longus (ECRL) and the extensor carpi ulnaris (ECU) muscles in man was studied during wrist extensions in which the two muscles acted as synergists. Intramuscular recordings were obtained using special quadrifilar needle electrodes. Isometric wrist extensions at 20-30% of the maximal effort were studied. The electromyographic (EMG) signals were decomposed into the individual motor-unit action potential trains comprising the signal. The interaction among motor units were characterized by the estimated time-varying mean firing rate and the cross-correlation between the time-varying mean firing rates of pairs of motor units. Pairs of motor units within each muscle as well as pairs of motor units across the muscles were considered. In-phase common fluctuations, termed common drive, were observed in the mean firing rates of motor units within each muscle, consistent with earlier work on other muscles. Common fluctuations were also observed between the firing rates of ECU and ECRL motor units albeit with a variable phase shift. The existence of common drive across synergistic muscles was interpreted as implying that the CNS considers the muscles as a functional unit when they act as synergists.

Surface EMG models: properties and applications

After a general introduction on the kind of models and the use of models in the natural sciences, the main body of this paper reviews potential properties of structure based surface EMG (sEMG) models. The specific peculiarities of the categories (i) source description, (ii) motor unit structure, (iii) volume conduction, (iv) recording configurations and (v) recruitment and firing behaviour are discussed. For a specific goal, not all aspects conceivable have to be part of a model description. Therefore, finally an attempt is made to integrate the 'question level' and the 'model property level' in a matrix providing direction to the development and application of sEMG models with different characteristics and varying complexity. From this overview it appears that the least complex are models describing how the morphological muscle features are reflected in multi-channel EMG measurements. The most challenging questions in terms of model complexity are related to supporting the diagnosis of neuromuscular disorders.

Cortical drives to human muscle: the Piper and related rhythms

During voluntary activity in humans, motor units are exposed to a number of descending drives that tend to synchronize motor unit activity at particular frequencies. In particular, the contralateral motor cortex drives muscle discharge in the beta (15-30 Hz) and Piper (30-60 Hz) bands. The cortical activity in these bands is task-specific, somatopicly distributed and generally precedes muscle discharge by an interval appropriate for conduction in fast pyramidal pathways. Coherence between cortex and muscle in the beta band is found during isometric contractions of weak to moderate strength. Thus oscillations within the beta band seem to coincide with a stable, relatively immutable state--a free running mode of the motor cortex that may maintain stable motor output with a minimum of computational effort. In contrast, coherence between cortex and muscle in the Piper band is most evident during strong isometric contractions or during movement. Demands on the motor cortex are likely to be greater and more mutable under these circumstances. Synchronisation in the gamma band may provide a means of binding together those particular, often spatially distributed, cortical elements involved in movement execution under conditions that vary from moment to moment and require some attention. Mechanisms both intrinsic and extrinsic to the cortex determine the pattern of rhythmic cortical activity. The basal ganglia have a pivotal role in this regard, and inadequate output from these nuclei leads to a disappearance of the beta and Piper drives to muscle. This may in turn contribute to slowness and weakness in Parkinson's disease.

The unilateral and bilateral control of motor unit pairs in the first dorsal interosseous and paraspinal muscles in man

1. The discharges of two motor units were identified in an intrinsic hand muscle (first dorsal interosseous, FDI) or an axial muscle (lumbar paraspinals, PSP) in ten healthy subjects. Each motor unit was situated in the homologous muscle on either side of the body (bilateral condition) or in the same muscle (ipsilateral condition). The relationship between the times of discharge of the two units was determined using coherence analysis. 2. Motor unit pairs in the ipsilateral FDI showed significant coherence over the frequency bands 1-10 Hz and 12-40 Hz. Motor units in the ipsilateral PSP were significantly coherent below 5 Hz. In contrast there was no significant coherence at any frequency up to 100 Hz in the bilateral FDI condition and only a small but significant band of coherence below 2 Hz in the bilateral PSP condition. 3. Common drive to motor units at frequencies of < 4 Hz was assessed by cross-correlation of the instantaneous frequencies of the motor units. A significantly higher coefficient was found in the ipsilateral FDI, ipsi- and bilateral PSP compared with shifted, unrelated data sets. This was not the case for the bilateral FDI condition. 4. The presence of higher frequency coherence ( > 10 Hz) in the ipsilateral FDI condition and its absence in ipsilateral PSP is consistent with a more direct and influential cortical supply to the intrinsic hand muscles compared with the axial musculature. The presence of low frequency drives (< 4 Hz) in the bilateral PSP condition and its absence in the bilateral FDI condition is consistent with a bilateral drive to axial, but not distal, musculature by the motor pathways responsible for this oscillatory input.

Occurrence of widespread motor-unit firing correlations in muscle contractions: their role in the generation of tremor and time-varying voluntary force

The firing behavior of motor units (MUs) of the first dorsal interrosseus muscle of the hand was examined during both constant-force and varying-force (sinusoidal or broadband random variations) isometric contractions in healthy adults. The emphasis was on the analysis of MU synchrony with an efficient and sensitive method. In static contractions, widespread and strong MU firing correlations, with the MUs in phase, were present at the frequency of muscle tremor, when the tremor was regular (narrowband) and large. MU correlations could also exist in contractions where the tremor of a subject was irregular (broadband) overall, but they were generally weak. These correlations were at the frequency of the subject's regular tremor, and the corresponding distinct tremor component was sometimes discernible within the broad tremor-band. In contrast, the MUs did not show any such correlations in the case of purely irregular and small tremor. On the basis of these observations, it is concluded that the rhythms in the force contributions of the last- recruited, large MUs, which fire near their threshold rate, compose the broadband frequency content of physiological muscle tremor in every contraction. Within this band, there is an additional distinct tremor component when MU correlations are present. For widespread and strong MU correlations, this component dominates and constitutes the observed regular tremor. In dynamic contractions, the firing of all MUs was modulated in the frequency band of both the sinusoidal and the complex variations of the force. The MU modulations showed a time-lead over the force variations and were strongly correlated both to these variations and among themselves. Thus widespread and strong correlations of MU firing modulations seem to provide a mechanism for generation of time-varying voluntary force, under general dynamic conditions. Finally, when regular tremor was present in dynamic contractions, widespread and fairly strong MU correlations also existed at the tremor frequency. It is concluded that at least two mechanisms can cause widespread MU synchrony, and they can act in parallel. They involve two types of correlated inputs to the alpha-motoneurons (presumably from the muscle spindles and the cortex), whose effects combine at the level of the membrane potential of the cells.

Relationship between motor unit short-term synchronization and common drive in human first dorsal interosseous muscle

We assessed the strength of motor unit (MU) short-term synchronization and common fluctuations in mean firing rate (common drive) in the same pairs of MUs in order to evaluate whether these features of voluntary MU discharge arise from a common mechanism. Shared, branched-axon inputs, with the most important being widely divergent monosynaptic projections to motoneurons from motor cortical cells, are regarded as the principal determinants of MU short-term synchronization. It is not known to what extent these synaptic inputs are responsible for common drive behaviour of MUs. MU spike trains from 77 pairs of concurrently active MUs in first dorsal interosseous muscle of 17 subjects were discriminated with the high reliability needed for common drive analysis. For each MU pair, the data used for comparison of the two analyses of correlated MU discharge came from a single trial (1-5 min duration) of isometric abduction of the index finger. Linear regression revealed a weak, significant positive correlation between the strength of MU short-term synchronization and the strength of common drive in the MU pairs (r2 = 0.06, P < 0.05, n = 77), which was slightly stronger when MU pairs with broad synchronous peaks (> 20 ms) were excluded (r2 = 0.09, P < 0.05, n = 63). These data suggest that less than 10% of the variation in the strength of common drive exhibited by pairs of MUs could be accounted for by differences in the strength of MU short-term synchronization. These two phenomena are therefore likely to arise predominantly from separate mechanisms. At least under these task conditions, the widely divergent, branched-axon inputs from single corticospinal neurons which are important in the generation of MU short-term synchronization play only a minor role in the production of common drive of MU discharge rates.

Control of motor units in human flexor digitorum profundus under different proprioceptive conditions

1. Changing the posture of the human fingers can functionally 'disengage' the deep finger flexor muscle from its normal action on the terminal phalanx of the fourth (or third) finger. This enables the activity of the muscle to be studied both with and without its normal proprioceptive inputs. 2. Spike trains of long duration from pairs of concurrently active motor units in this muscle were recorded in both the engaged and disengaged hand postures. Subjects voluntarily kept one of the motor units (the 'controlled' unit) discharging at the same target frequency in both postures. The strength of short-term synchrony, the strength of common drive, and the variability of discharge of these pairs of motor units were determined in both postures. 3. All subjects reported that the effort required to activate the motor units in the disengaged hand posture was substantially greater than in the normal engaged posture. 4. Short-term synchrony, which is a function of common corticospinal inputs to pairs of motor units, was similar in both hand postures. However, the strength of common drive was significantly decreased when the muscle was disengaged. Although the neural substrate for common drive is not known, this observation suggests that proprioceptive feedback is involved either directly or indirectly. 5. Although the discharge rate of the 'uncontrolled' motor units increased when the muscle was disengaged, the variability of discharge of these and the 'controlled' motor units increased significantly. This supports the idea that the precision with which fine motor tasks can be performed is improved when proprioceptive feedback is intact.

Physiology and interpretation of the electromyogram

The purpose of this review is to consider some issues in the interpretation of the electromyogram (EMG) and to discuss current areas of controversy regarding use of the EMG. We consider the underlying physiology and origin of the EMG signal and offer an abbreviated discussion of measurement issues and selected factors that affect the characteristics of the EMG signal. We discuss many of the problems affecting interpretation, including normalization, crosstalk, and issues specific to contraction. In the final section, we consider topics of current interest in electromyography, such as muscle fatigue, task specificity, multichannel representations, and muscle fiber conduction velocity. We present, in addition, alternative analysis techniques. This review should interest researchers and clinicians who seek to obtain the valuable information inherent in the EMG while respecting the potential sources of variance and misinterpretation.